The present invention relates to a telecommunications system comprising base stations, for example of the Digital European Cordless Telecommunications (DECT) type, managing picocells and interconnected by a local area network, in particular an Internet Protocol (IP) local area network.
A local area network using the IP as its transmission protocol cannot synchronize DECT base stations with each other and thereby provide handover, i.e. continuity of call transmission when a mobile terminal such as a DECT mobile telephone moves from one cell to another. This is known in the art.
The document WO-01-78246 describes a telecommunications system comprising a plurality of base stations using the BLUETOOTH technology for short-range radio communication, the base stations each covering a picocell and being interconnected by a local area network.
The above document discloses a method of synchronizing base stations including the following steps:                a step of coarse synchronization of each base station adjoining an active base station, i.e. one communicating with a mobile terminal, the active base station sending a coarse synchronization signal to the adjoining base stations via the local area network,        a step of sending a reference clock, the active base station periodically sending the reference clock at a high power level in the context of the frequency hopping aspect of the BLUETOOTH technology, and        a step of fine synchronization of an adjoining base station when the mobile is detected in the intersection area between the active picocell and the adjoining picocell, by listening either to the active base station or to the moving mobile station.        
The base stations are synchronized by tricking a moving mobile terminal into believing that it still in the same picocell so that it retains both the synchronization and the frequencies of the original active base station with the associated frequency hopping law.
This synchronization method is complex and must be executed “dynamically”, i.e. immediately handover is required.
Furthermore, to effect this synchronization, it is necessary to provide a set of supplementary radio means in each base station adapted to be configured when a mobile terminal is moving. These means must be available for the moving mobile terminal and cannot be used for other tasks. This impacts on the cost of the configurations to be implemented. In particular, this type of technique is not suitable for high traffic levels.
Users of a transmission system, for example a professional transmission system, are committed to maximizing the return on the investment in their infrastructures.
U.S. Pat. No. 5,519,759 describes a method of synchronizing a plurality of base stations in a DECT wireless telecommunications system comprising base stations each covering a picocell and interconnected by a local area network comprising a plurality of switches. The method includes a system initialization phase including a step, carried out within each group of base stations covering adjoining picocells, of organizing the base stations hierarchically. This hierarchical organization step consists in a first base station of the group concerned broadcasting a synchronization signal, the other stations of the group that are capable of receiving the signal receiving it and measuring its power, transmitting the results of the power measurements to a central server, comparing the results to determine the station that received the highest power, instructing that adjoining station to synchronize to the first station and then to send in its turn a synchronization signal, and repeating the above process until all the stations constituting a group have been synchronized in succession.